Wire saw technology is used in a variety of industrial applications. They have been used to cut rock in the mining industry. Conventional wire saws generally have abrasive beads strung along a wire, cable or rope threaded through a central bore of each bead. The abrasive grains are embedded into the outer surface of the beads and the beads are longitudinally displaced on the wire by spacers. See for example, U.S. Pat. No. 5,377,659 to Tank et al. Also, U.S. Pat. No. 5,383,443 to Buyens provides an improvement to a beaded wire saw in which the beads are mounted eccentrically on the wire.
China Grinding Wheel Co., Taipei, Republic of China, offers a beaded wire saw that uses diamond grains brazed to the bead. These beads are available under the tradename Kinik.RTM. DiaGrid.RTM. Pearls for use in cutting construction material such as marble, serpentine, granite and concrete.
The slicing of ceramic, especially blocks of single crystal silicon, occasionally referred to herein as a boule of silicon, to make thin wafers is very important for the microelectronics, optics and photovoltaic industries. The precision of cut is important to produce wafers that are flat to high dimensional tolerance. Traditionally, ceramic wafers have been made by sawing the boule with an unusual grinding wheel having abrasive bound to the inner diameter of a central bore. Such "inner diameter sawing" allows for extremely precise cutting but is limited to slicing only one wafer at a time.
Recent wire sawing has been applied to ceramic wafer production. Increased productivity can be obtained by using a long wire saw strung in such a way as to make many passes across the length of the boule thereby slicing many wafers simultaneously. The work piece is of very high quality and therefore, even slight waste of raw material can be very costly. Prior wire saw technology thus involves using a plain metal wire and loose abrasive grains applied at the interface between the wire and the boule.
Conventional beaded wire saws generally are not suitable for precision grinding demanded by wafer slicing. The beads increase the effective thickness of the tool which cuts too wide a kerf through the work piece. The mass of work piece removed by the beaded saw can be many times that of a single wafer. The use of spacers and sleeves further complicates fabrication of beaded saws.
Another technique that might reduce the kerf of a wire saw for wafer slicing involves electrochemical deposition of abrasive directly onto the wire substrate. Electrochemical deposition generally calls for placing an electrically charged wire in a bed of abrasive particles in an oppositely charged liquid solution of a metal compound. As metal precipitates on the wire, it captures abrasive particles within a thin metal layer and thereby binds the abrasive to the wire. For example, U.S. Pat. No. 5,438,973 to Schmid et al., discloses blades having diamond abrasive particles fixed in nickel plating to a cutting surface of a tear-drop cross section stainless steel wire core.
Wire saws made by electrochemical deposition have the principal shortcoming that there is no chemical bond between the abrasive and the deposited coating. During operation, the outer surface of the thin layer is soon worn away, and the abrasive particles easily dislodge from the wire when less than about half of the deposited metal is eroded. Hence, the saw becomes ineffective prematurely, i.e., before the abrasive particles become dull. Plated metal can also debond from the wire under cyclic load.
Another shortcoming of electrochemically deposited wire saws is that they are expensive to manufacture. The mass of abrasive in the bed should be far in excess of that which actually ends up on the wire. Of course, superabrasive particles are quite expensive, and the need to maintain an inventory of particles in the bed raises the cost. Furthermore, control of the abrasive particle distribution on the wire is not practicable.
It is desirable to have a superabrasive wire saw that has a small cross section dimension, especially for cutting thin sections such as ceramic wafers. A superabrasive wire saw that has long service life and which is simple and relatively inexpensive to make is also needed. It is further desired to have a process for making such a wire saw that provides precise and sensitive control of abrasive distribution on the wire.
Accordingly, the present invention provides a wire saw comprising a metal wire, and superabrasive grains affixed to the wire by a brazed metal bond, wherein the grains are preferably disposed upon the surface of the wire with a preselected surface distribution.
There is further provided a process for making a wire saw comprising the steps of:
(a) providing a paste comprising a metal brazing composition; PA1 (b) coating the surface of a wire with a layer of the paste; PA1 (c) depositing on the layer of paste a layer of abrasive grains; PA1 (d) heating the wire in an inert atmosphere to a temperature and for a duration effective to fuse the brazing composition; and PA1 (e) cooling the wire thereby brazing the grains to the wire.